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DESIGN EARTHQUAKES FOR ITER IN EUROPE AT CADARACHE

Comparison of the semi-empirical methods used according to the French regulation and the seismic probabilistic assessment of the site

Jean-Philippe Girard1, Gottfried Grünthal2, Marc Nicolas3 and EISS Team

1 Direction de l‘Énergie Nucléaire, CEA Cadarache, 13108 Saint-Paul-lez-Durance, France 2 GeoForschungsZentrum Potsdam, Germany 3 CEA-DAM Íle de France, 91680 Bruyères-le-Châtel, France Phone: +33 4 42 25 66 74 œ Fax: +33 4 42 25 25 00 œ Email: jean-philippe.girard@ cea.fr

Abstract

Input requirements and assumptions for ITER consider that an infrequent, severe earthquake (called SL-2) which, although unlikely to occur during the lifetime of the facility, is assessed to demonstrate adequate protection of the public. This earthquake is assumed to have a return period of 10,000 years. An investment protection level or inspection level (where all Structures, Systems and Components are safe) with a peak ground acceleration (pga) at 0.5 m/s2 is also considered. As a basis, orders of magnitude of consequences, if no countermeasures were taken, are given.

The European site proposed for ITER is situated in the South of France, 40 km North-East of Aix-en-Provence, in a low to moderate seismic area according to the Global Seismic Hazard Map (GSHAP Group 1999). The tokamak building would be implemented on good bedrock made of limestone with a shear wave velocity of over 1,300 m/s.

Four aspects are discussed: regulation, implementation of this regulation for the proposed site (site geology, tectonic, seismotectonic), a probabilistic seismic hazard assessment of the site has been performed using a methodology which considers uncertainties (this work has been developed by the team which had contributed to large parts of the GSHAP map study), finally the fulfilment of the requirements and assumptions are discussed, according to IAEA guides for instance.

As a conclusion of the studies the main characteristics of the Cadarache European site are discussed: as a reference the standardized 475 year mean return period hazard (i.e. 10 % of chance to occur in 50 years); 10,000 year return period in accordance with the ITER assumption. Preliminary studies have shown that the European site proposal will ensure a low level of project risk with respect to the seismic hazard.

Keywords: ITER, Cadarache, seism, geology

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Input requirements and assumptions for ITER consider that an infrequent, severe earthquake which, although unlikely to occur during the lifetime of the facility, is assessed to demonstrate adequate protection of the public. This earthquake is assumed to have a return period of 10,000 years. An investment protection level or inspection level (where all structures, systems and components are safe) is also defined with a peak ground acceleration (pga) of .05 g (or 0.5m/s2).

1. Radioactive inventory and potential risk and impact

The ITER tritium inventory is in the order of a kilogram of tritium. The impact of such an inventory on nearest population could be in the order of few tens of mSv to the Sievert according to the chemical form of the tritium (gas or water) if no confinement would be designed to prevent releases of the tritium. 1 to 10 mSv is the upper limit of design values usually consider for infrequent severe accident consequences. Though if no other contamination sources were considered (i.e. radioactive with irradiated dust, or chemical with beryllium), ITER would have to be designed to confine tritium during major accident such as severe earthquakes.

2. Seismic Hazard Assessment at the proposed European ITER Site

The probabilistic seismic hazard assessment analysis for the proposed ITER site at Cadarache, France, is described. Uncertainties in the input parameters lead to fractiles reflecting the uncertainty of the output hazard. According to the IAEA SL-1 and SL-2 standards, median hazard values at annual probabilities of occurrence of 10-2 and 10-4 years, respectively, are given.

2.1. Seismicity data

For the probabilistic calculation of the seismic hazard at Cadarache the French earthquake catalogue —Sisfrance“ [1], calibrated in terms of moment magnitudes Mw, served as the basis for the study. The spatial distribution of epicentres in the region of southern France (including parts of Italy) is shown in Figure 1 (see [2] and [3]).

2.2. Input parameters and computation algorithm

The uncertainty of the input parameters used for a probabilistic hazard calculation are considered in the so-called —logic tree“ algorithm. For each input parameter, several values or models are entered with their relative weights. The seismic source zone model proposed in Martin [4] et al. (2001) was adopted, dividing France into 52 seismic source zones. The zones around Cadarache are shown in Figure 2. The source zone model was modified with respect to the Durance fault, which is considered as separate. Seismicity parameters calculated for each zone for the logic tree input are the characteristics (a- and b-values) of the Gutenberg- Richter frequency-magnitude relation based on complete data in each size class, maximum expected magnitudes and representative focal depths [5]. The input a- and b-values are given with ± one standard deviation to account for the uncertainty. The maximum magnitude values are obtained with a technique modified after Cornell (1994) [6] and Coppersmith (1994) [7] based on data from globally distributed stable continental regions. Five representative values of the focal depth distribution are given for each source zone. Samples of frequency- magnitude relations, maximum magnitude distributions and focal depth sets are given in Figure 2. The obtained magnitude values are well above the respective maximum observed

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magnitude, e.g. for two of the zones in Figure 1, the highest maximum values are more than two magnitude units above the observed magnitude. Since no sufficient numbers of strong- motion recordings exist for southern France, attenuation relations from similar tectonic regimes and with similar subsoil characteristics were applied for the Cadarache calculations. Since both, extensional tectonics and strike-slip movement are prevailing in the surrounding of the site, attenuation relations for the peak ground acceleration by Spudich [8] et al. (1999) for extensional regimes (33 % weight) and by Boore [9] et al. (1997) assuming strike-slip mechanism (33 %) and not specified mechanism (33 %) were applied. According to the measured shear wave velocities at the Cadarache site all relations are for rock type conditions.

3. Results Seismic Hazard Assessment

Each of the branches (combinations) of the logic tree gives hazard solutions for a range of annual probabilities of occurrence for the proposed European site. Fractiles of the solutions of all branches are selected and plotted in Figure 2, the median 50 % curve and the 16 % and 84 % fractiles are shown. ITER seismic design specifications are based on the median hazard for mean return periods of 102 and 104 years. The obtained median hazard values for Cadarache are respectively 0.67 m/s2 and 2.26 m/s2. From published seismic hazard maps in recent domestic assessments for Cadarache (Martin et al., 2001), hazard values in the ranges 0.5-0.7 m/s2 for a mean return period of 100 years and 1.5-2 m/s2 for 1975 years mean return period were derived. These values are in good agreement with the findings of the present study (Figure 2).

4. Site ground characteristic

The European proposed site for ITER is situated in the South-East of France on a limestone bedrock close to the confluence of the Durance and Verdon Rivers in Provence. Extensive drillings of the proposed site leads to select an area with dense limestone (density = 2.5 t/m2) up to the natural level to implement the tokamak building and the other nuclear and safety building. Sixty drillings from 20 to 60 metres on the whole expected ITER building platform, a 200 metre deep drilling is available nearby on the Cadarache Centre. It has been shown through seven seismic refraction profiles that the limestone is homogeneous (little fractured) and free from any karstic cavity and from any marly or clayey beds. The characteristics of the ground have been measured and largely fulfil the requirement of ITER in terms of soil bearing capacity (400 t/m2 for a design load of 100 t/m2), in case aseismic bearings would be used no differential settlement have to be feared. The shear wave velocity (which is a criteria of the ground in terms of classification of soil vs. amplification of seismic hazard) has been measured trough three cross-holes technique on the first 30 metres under the zero level of the nuclear building and leads to mean on 30 first metres). W ith a mean over the 30 first metres under the tokamak basemat of 1,350 m/s, these measurements leads to classify the soil as good hard bedrock (on a scale of three types of soil, poor, medium and good) the threshold for —good“ being 800 m/s according to the French regulation and 1,100 m/s according to the IAEA standards.

5. Regulation to apply for ITER in Europe and in France

According, among others, to the tritium inventory ITER will be a so-called —Installation Nucléaire de Base“. A prescriptive rule for the seismic hazard of such facility is available and has to be fulfilled. This rule has recently been updated (2001). The standard procedure is to integrate the elements of the seismological (instrumental seismicity, historical and

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prehistorical data), geophysical and geological database to construct a regional seismotectonic model consisting of a discrete set of seismogenic structures and zones of diffuse seismicity. The maximum potential earthquake of each seismogenic structure and each zone of diffuse seismicity should be assumed to occur at the point of the structure (or zone) closest to the site area. A regional attenuation relation should be used to determine the ground motion that each of these earthquakes would cause at the site. The SMHV (Séisme Maximum Historiquement Vraisemblable) is (are) the earthquake(s) to occur the maximum ground motion at the site area. The local conditions (site effects, capable faults… ) are also studied. The design earthquake SMS (Séisme Majoré de Sécurité) corresponds to the SMHV at the same location with a 0.5 increment of magnitude for uncertainty and conservatism. The characteristics of the ground motions corresponding to SMHV and SMS levels should be expressed in terms of response spectra. The contribution of multiple seismic sources is represented by an envelope. The response spectra are standard and include in the rule. The response spectra database take account 1D geological site effects with 3 classes. Hard rock corresponds to Vs30 > 800 m/s.

6. Implementation of the seismic hazard regulation for ITER at Cadarache

The seismicity of southern France probably results from the convergence between Africa and Eurasia which proceeds at a rate of 0.8 cm/year at the Provence longitude. The potentially seismogenic structures delimit a large panel of Mesozoic cover. It includes E-W compressive faults (Mont Ventoux, Montagne de Lure, Lubéron, Costes and Trévaresse) and N-E left- lateral strike slip (Durance, Nîmes and Cévennes). The major historical earthquake of Provence (over the last 1,000 years) is associated with Costes Trévaresse structure (Lambesc, 1909, M 5.5). The Durance fault is associated with only few and moderate historical seismic events (M 5-5.3), but paleoseismic evidence for larger earthquakes (M 6.5-6.7) were found on subsidiary fault near Manosque. For the zone of diffuse seismicity, the maximum magnitude observed is 4.9. Only one earthquake M ≥ 6 occurred within 200 km over the last 1000 years. In application of the French safety seismic hazard rule, the SMHV for ITER area corresponds to an M = 5.3 event located at the Durance Fault at a hypocentral distance of 7.1 km (minimum distance between this fault and the ITER site). The corresponding SMS is an M = 5.8 event at the same position. The length and the displacement associated of the Durance fault prehistoric event about 9000-26000 years ago and the Gutenberg-Richter analysis conduct to consider at the SMS level a M = 7 earthquake at a hypocentral distance of 18.5 km. The SMS design spectrum for the Cadarache (Figure 3) site is finally determined from the median ground motion for hardrock expected for two events. An M = 5.8 earthquake located 7.1 km and an M = 7 earthquake located 18.5 km. This spectrum has an estimated return period of 13,000 years in good agreement with the IAEA level SL-2 requirement.

7. Discussion and conclusion

Based on an acceptable methodology according to the IAEA safety recommendation, the safety level seismic hazard for ITER fulfils the requirements and assumptions. W hen the semi-empirical method is compared to the seismic probabilistic assessment a margin in terms of return period ascertains that an infrequent severe earthquake defined as the SMS in the French regulation has a return period of more than 10,000 years. As a basis for comparison to other sites the standard 10,000 years return period hazard is estimated to a peak ground acceleration (pga) of less than 2.3 m/s2. At least the inspection level acceleration (pga 0.05 g) should occur less than once in the facility life. As a reference the standardized 475 year mean return period hazard (i.e. 10 % of chance to occur in 50 years) is close to 1 m/s2. Thus, the construction of ITER at Cadarache, in Europe, as a nuclear facility according to the French

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regulation, with demonstrated and validated seismic hazard warranties a low level of project risk from this point of view. Position of nuclear buildings has been optimized to reduce seismic hazard to consider at basemat. Moreover, in order to minimize constrains on the equipment, it is proposed to use aseismic bearings for the tokamak building [11].

References

[1] Sisfrance catalogue: http://www.sisfrance.net [2] Baroux, E., Béthoux, N. and Bellier, O., —Analyses of the stress field in south-eastern France from earthquake focal mechanisms“, 2001, Geophys. J. Int., 145, 336-348 [3] Béthoux, N., Ouillon, G. and Nicolas, M., —The instrumental seismicity of the western Alps“, 1998, Geophys. J. Int., 135, 177-194 [4] Martin, Ch., Combes, Ph., Secanell R., Lignon G., Carbon, D., Fioravanti, A., Grellet, B., —Revision du Zonage Sismique de la France“, 2001, Rapport n°GTR/MATE/0701 - 150Av3, 251 pp. [5] Grünthal, G. and W ahlström, R., —Sensitivity of parameters for probabilistic seismic hazard analysis using a logic tree approach“, 2001, J. Earthq. Engrg. 5, 309-328 [6] Cornell, C. A., —Statistical analysis of maximum magnitudes, in: The Earthquakes of Stable Continental Regions, Vol. 1, 1994 (EPRI) TR-102261-V1, pp. 5-1-5-27 [7] Coppersmith, K. J., —Maximum earthquake assessment, in: The Earthquakes of Stable Continental Regions, Vol. 1“, 1994 (EPRI) TR-102261-V1, pp. 6-1-6-24 [8] Spudich, P., Joyner, W .B., Lindh, A.G., Boore, D.M., Margaris, B.M. and Fletcher, J.B., —SEA99: Ground motion prediction relation for use in extensional tectonic regimes“, 1999, Bull. Seism. Soc. Am., 89, 1156-1170 [9] Boore, D.M., Joyner, W .B. and Fumal, T.E., —Horizontal response spectra and peak acceleration from North American earthquakes“, 1997, Seism. Res. Lett., 68, 128-153 [10] Lyraud, C., et al. —Readiness of Cadarache for starting ITER construction“, 23rd SOFT, 2004, Venice, Italy [11] Fardeau, A., et al. —Studies for Site preparation for ITER construction“, 23rd SOFT, Venice, Italy

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List of figures

Figure 1: Seismic source zones and seismicity around Cadarache with example of Gutenberg-Richter magnitude-frequency relation, maximum magnitude distribution curves (max. observed Mw at vertical red line; maximum magnitude values at arrows œ see Grünthal and W ahlström, 2001 for details) and input focal depth set

Figure 2: Seismic hazard curves for peak ground acceleration (pga) (median (50 % fractile), 16 % fractile and 84% fractile) for the site Cadarache

Figure 3: Seismic hazard spectrum for ITER at Cadarache

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Figure 1: Seismic source zones and seismicity around Cadarache (M w ≥ 3.5, complete with M w ≥ 5 since 1700, in red one event M w=6), example of Gutenberg-Richter magnitude-frequency relation, maximum magnitude distribution curves (max. observed M w at vertical red line, maximum magnitude values at arrows œ see Grünthal and W ahlström, 2001 for details) and input focal depth set

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Figure 2: Seismic hazard curves for peak ground acceleration (pga) (median (50 % fractile), 16 % fractile and 84 % fractile) for the site Cadarache

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Figure 3: Seismic hazard spectrum for ITER at Cadarache

Cadarache

1 ) g (

n o i t a r e l

e 0.1 c c a - o d u e s P

0.01 0.1 1 10 100 Frequency (Hz)

SMS Rock M=5.8 Distance 7.1 km - Plant

Paleoseism Rock M=7 Distance 18.5 km - Plant 5% Damping

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